This invention relates in general to electronic circuits for operating arrays of light emitting diodes (LEDs). In particular, this invention relates to an improved method of decreasing visible flickering effects in an LED array that is operated by pulse width modulated control signals.
LEDs are commonly used as sources of light in a variety of applications, including vehicles (such as for headlights, taillights, signal lights, and brake lights, for example) and other devices. Typically, each of such LEDs in an array is selectively connected through a switching element to a source of electrical energy. When the switching element is closed, the associated LED is connected to the source of electrical energy. As a result, electrical current flows through the LED, causing it to emit light. Conversely, when the switching element is opened, the associated LED is not connected to the source of electrical energy. As a result, electrical current does not flow through the LED, causing it to not emit light. The closing and opening of each switching element is usually controlled by an electronic controller, which generates electrical control signals to the switching element. In response to such electrical control signals, the switching element is selectively closed and opened to operate the associated LED in a desired manner.
The brightness of light emitted by the LED is directly related to the amount of electrical current that flows therethrough. The amount of electrical current that flows through the LED can be controlled by varying the duty cycle of a pulse width modulated (PWM) electrical control signal supplied to the switching element. When the duty cycle of the PWM electrical control signal is relatively short, a relatively small amount of electrical current flows through the LED, and the LED will emit a relatively low brightness of light. Conversely, when the duty cycle of the PWM electrical control signal is relatively long, a relatively large amount of electrical current flows through the LED, and the LED will emit a relatively high brightness of light.
In some instances, it may be desirable to change the amount of the brightness of light emitted by the LEDs. For example, in the context of a vehicle headlight, it may be desirable to dim the amount of the brightness of light emitted by the LEDs from a relatively high brightness of light to a relatively low brightness of light when the presence of oncoming traffic is detected. To accomplish this, the duty cycle of the PWM electrical control signal is transitioned from an initial relatively large magnitude to a final relatively small magnitude.
It is known that transitioning between different levels of brightness of light can result in visible flickering of the LEDs if the following conditions are present: (1) the LEDs are transitioning in the same manner (e.g., are all dimming); (2) the LEDs are physically adjacent or near each other in an LED array; and (3) the LEDs have different dimming rates of change. Flickering is caused by modifications of brightness in relation to the modified brightness of neighboring LEDs. One solution for minimizing these undesirable flickering effects would be to increase the frequency at which the pulses of the PWM electrical control signal are sent to the switching element, thereby increasing the speed at which the switching element is to be opened and closed. However, increasing the frequency at which the pulses of the PWM electrical control signal are sent to the switching element can undesirably increase the cost of the electronic controller. Thus, it would be desirable to provide an improved method of dimming LEDs operated by a pulse width modulated control signal that reduces visible flickering effects, while being simple and inexpensive to implement.